The present invention relates to an identification system composed of a proximity powered and coded portable device or unit and a corresponding energization and interrogation device which generally has a fixed installation. The portable unit may be in the form of a tag or card, and for convenience is sometime referred to herein as a tag or transponder. The present invention thus relates to a system wherein a fixed installation sends out energy to activate a portable responsive device which would ordinarily be carried by a movable person or thing and which is in the vicinity or proximity of the fixed installation, and the device so energized sends out a coded signal to be picked up by a receiver. The receiver in turn, activates some system functions, for example, to provide access to a controlled area, to keep track of the movable person or thing, or to perform similar purposes.
Proximity identification technology systems are becoming a well known and important means of identification by which coded tags are read using a non-contact means of communication. A proximity identification system consists of four major components: a tag, which when excited, transmits stored identifying data, an exciter which transmits a signal to the tag to activate same, a signal receiver for detecting and decoding data transmitted by the tag, and a coupling mechanism by which the activating or exciting signal is transmitted to the tag and the stored data is transmitted to the receiver.
In the known proximity identification systems, power for the tag is generally provided by the activating signal which, by means of an electromagnetic coupling mechanism, transmits sufficient energy to the tag to supply power for its operation. However, various different coupling mechanisms by which data is transmitted back to the receiver are utilized. One of the more common coupling mechanisms for data transmission uses low frequency magnetic coupling. A more recently introduced technology of paramount importance uses an electric field coupling mechanism for data transfer. In this later coupling mechanism, the frequency of operation is likewise low, in the sense that a wavelength is much longer than the physical dimensions of the radiating structure(s). Due to this low frequency of operation, quasistatic or near field conditions exist.
As indicated above, the most common type of proximity identification system utilizes an electromagnetic coupling mechanism for both the exciting (power) signal and the data signal, with the data transmission frequency being a subharmonic or submultiple of the exciter frequency to provide isolation between the two signals. The use of such a coupling mechanism provides a number of advantages and disadvantages.
The advantages of electromagnetic coupling are as follows:
1. The coupling mechanism between the tag and the receiver is inductive in nature, arising from mutual coupling between the two, and the source impedance in the near field is very low. Thus, there is no appreciable shielding of the coupled signal due to conductive objects such as would be caused by covering the face of the tag with, for example, the hand of the user while introducing the tag into the exciter field; and PA1 2. There is negligible reduction in range when the tag has no nearby ground reference. PA1 1. Substantial power is required to transmit the coded information back to the fixed receiver. PA1 2. The magnetic field, and thus the received signal, drops off as the cube of the distance from the source. PA1 3. The data signal transmitted from the tag to the receiver is coupled by the same mechanism used to couple the exciter signal to the tag (magnetic), which brings about a difficulty due to the direct or harmonic relationship between the data signal and the exciting (powering) signal resulting in difficulty in extracting the much weaker information signal from the excitation signal. PA1 1. Negligible power is required to retransmit the coded information from the tag to the receiver. PA1 2. The electric field coupling antenna is small and easily integrated onto a small surface such as a card, causing this coupling mechanism to be more practical in terms of cost, simplicity, and increased reliability as opposed to an electromagnetic coupling mechanism. PA1 3. The received signal drops off as the square of the distance from the source, as opposed to the cube of the distance realized with electromagnetic coupling. PA1 1. It is more important for the body or object bearing the tag to be physically connected to the tag for a good read range (body being referenced to ground potential), since doing so increases the capacitive coupling between the tag and the receiver due to the direct coupling path in one direction through the body in question with respect to ground reference. PA1 2. The coupling mechanism from the tag to the receiver, being capacitive in nature and of a high impedance in the near field, is easily shielded by nearby grounded and conductive objects, such as when the front face of the tag (card) is covered with the hand of the user while introducing the tag into the exciter field.
This coupling method is very attractive in applications where the tag is to be placed on or in close proximity to shielding bodies, or such as when the receiver loop or antenna is to be buried for sensing vehicles, so that it is subjected to various ground conditions as well as rain, mud, etc.
The main disadvantages of electromagnetic field coupling are as follows:
The disadvantages of the above mentioned proximity identification system using only electromagnetic field coupling are substantially overcome and additional advantages provided by a system such as disclosed in commonly assigned U.S. Pat. No. 4,818,855 issued Apr. 4th, 1989 which, while using electromagnetic coupling for transmission of the exciter (power) signal, transmits the coded information back to the fixed receiver via an electric field coupling mechanism. The most important advantage of this system is the provision of different coupling modes or mechanisms for the signal transmitting power, and the much weaker return coded signal, thus providing isolation between the two signals. Additional isolation between the two signals can be achieved by transmitting the coded information at a subharmonic of the frequency used to transmit the power. Additional advantages are as follows:
On the other hand, the disadvantages of electric field coupling are as follows: